It is known that a metal or synthetic resin article can be coated with a thermosetting resin by a process in which a solution of the resin in a solvent is applied onto a surface of the article, the applied resin solution layer is dried and, then, the dried resin layer is cured at an elevated temperature. The above-mentioned conventional thermosetting process is disadvantageous in that a large scale drying and curing apparatus must be used in order to evaporate the solvent from the resin solution layer and thermally cure the dried resin layer, that the solvent vapor generated in the drying and curing apparatus may pollute the atmosphere, and that a large amount of thermal energy is consumed for evaporating the solvent and for curing the resin layer.
In order to eliminate the above-mentioned disadvantages of the thermosetting process, a photosetting process was proposed. In the photosetting process, a liquid photosetting composition is applied onto a surface of the article and the applied photosetting composition layer is cured by the action of actinic rays, such as ultraviolet rays. The conventional photosetting composition contains, for example, an epoxy resin or a liquid polybutadien as a base component. However, the conventional epoxy resin type photosetting composition is disadvantageous in such features that the stability of the composition during storage is poor, that the photocuring reaction rate of the composition is low, that the adhering property of the composition to the article surface to be coated is poor, and that the resultant photocured composition exhibits a poor flexural strength and impact strength. On the other hand, the conventional liquid polybutadiene type photosetting composition exhibits a poor photosetting rate and adhering property, and the resultant photocured composition exhibits a poor hardness. Accordingly, neither the conventional epoxy resin type nor liquid polybutadiene type photosetting composition is proper as a coating varnish.
Japanese Patent Application Laying-Open Nos. 48-29886(1973) 49-98454(1974) and 51-37128(1976) disclose photosetting compositions containing, as a base component, an epoxidized polybutadiene or modified, epoxidized polybutadiene which exhibits both the properties of the epoxy resin and the liquid polybutadiene. However, these types of photosetting compositions still have the above-mentioned disadvantages of the conventional epoxy resin type and liquid polybutadiene type photosetting compositions. That is, these types of the photosetting composition have a poor storing stability, photocuring reaction rate and adhering property. Also, the resultant photocured composition exhibits a poor hardness and resistance to water.
Japanese Patent Application Laying-Open No. 51-37128(1976) discloses a photosetting composition containing, as a base component, another type of modified epoxidized polybutadiene which is prepared by reacting an epoxidized polybutadiene with acrylic or methacrylic acid, a photopolymerizable monomer and a photosensitizer. However, this photosetting composition has a relatively poor photocuring reaction rate and, therefore, a long time required to complete the photocuring reaction. Accordingly, this type of photosetting composition is unsuitable for practical use.